Nuclear magnetic resonance (NMR) spectroscopy is a widely used technique in physical and biological science for structural analysis of compounds. Generally, an aliquot of solution in an elongate precision-made sample tube is placed in a sample chamber located between poles of a powerful magnet and subjected to radio frequency exitation. The sample tube is customarily axially rotated to average out the magnetic field and radio frequency exitation.
Sample tubes for the instrument are generally made of glass and are available in a plurality of diameters generally ranging from about 1 mm to about 10 mm in diameter with lengths ranging from about 100 mm to about 8 inches. The most widely used tubes are about 5 mm in diameter and about 7-8 inches long. In use, the tubes are placed in a tube holder that is specifically designed to fit a particular manufacturer's instrument. The tube holder is a precision-made air turbine that spins the sample during the determination of the spectrum.
Since the tubes are elongate and spun during the determination of the spectrum, it is important that the tubes are substantially straight and are substantially coaxial with the tube holder. If the tubes are not substantially straight and coaxial with the tube holder, when the tubes are spun, there will be substantial “run-out” of the bottom portion of the tube that is between the excitation coils and the poles of the magnet. This run-out will degrade the resolution of the spectrum and is very undesirable.
Instrumentation manufacturers supply tube holders to fit specific size tubes. These tube holders, since they are precision-made air turbines, are costly and most users only inventory tube holders for the most commonly used size tubes. In many instances, researchers may only have small amounts of a compound available for testing. In these cases, the standard 5 mm/8 inch tube does not provide a satisfactory vessel for evaluation of these small amounts. As a result, tube manufacturers have developed several sample tubes and adapters to enable use of smaller diameter/low volume samples in standard sample holders.
Examples of these currently available specialized tubes and adapters are illustrated in FIGS. 4 and 5. FIG. 4 illustrates one approach wherein a larger diameter tube upper portion is axially joined to a smaller diameter lower portion that is placed in a standard tube holder so that the smaller diameter lower portion is positioned in the required position between the poles of the magnet. There are several difficulties with this type of tube. Joining the smaller diameter portion to the larger diameter portion requires precision glass blowing, and as a result, tubes of this type are expensive, difficult to clean and fragile.
FIG. 5 illustrates another approach to the problem. In this approach, a smaller diameter inner tube is positioned within a larger diameter outer tube by fitting an upper and lower adapter kit over the inner tube and placing the smaller diameter inner tube having the adapters thereon within the outer tube. While this approach is workable, fitting the adapters over the tube and correctly placing the smaller tube within the outer tube is tricky. It is easy to snap or misalign the small diameter tube during the fitment or removal of the adapter pieces, fitment or removal of the smaller tube from the larger tube and, additionally, these small pieces may easily be lost, subjected to dimensional changes and contamination.
Another approach offered by one instrument manufacturer utilizes a proprietary sample holder and clamping devices for their specific instruments. However, this device is usable only with spectrometers of that manufacturer, and, additionally, is not compatible with standard tubes or tube caps. There are reports from the field of incomplete sealing of tubes in this device because of tubes not being square or being chipped at the top resulting in loss of solvent during the determination of the spectrum.
Accordingly, there is still a need for a more universally applicable and simple to use adapter for allowing the use of smaller diameter tubes in an NMR sample holder designed for larger diameter sample tubes. Such an adapter is disclosed hereinbelow.